The present invention relates to apparatus useful in supplying a molten stream of metal to a spray-forming station.
More particularly it relates to an apparatus adapted for melting metal and for supplying a stream of molten metal to a gas atomization component of a spray-forming-apparatus.
It is well-known that spray-forming is a process which is carried out by developing a supply of liquid metal and by flowing a stream of the liquid metal into the path of the atomizing gas. The atomizing gas breaks up the single stream of molten metal into many tiny droplets. The spray-forming process involves the interception of the flight of these droplets before they turn to particles while in flight, and depends on the solidification of the droplets as they impact on a receiving surface. Spray-forming in this manner is a well-developed art and numerous articles can be formed from this spray deposit of this type process.
Normally the development of a liquid stream of molten metal requires that the molten metal be dispensed from a crucible either by pouring from the top of the crucible through a spout or by pouring from the bottom of the crucible through a suitable opening. The molten metal, particularly for the higher melting metals, requires that the crucible be formed of very high melting material and ceramic is the normal and natural choice of materials for such crucibles.
One problem which arises from the use of ceramic crucibles is that due to thermal shock or due to abrasion or some similar mechanism there is a possibility that a small ceramic particle will enter into the melt stream exiting from the crucible and will be incorporated in an article made by the spray-forming process. The problem which arises from the presence of such particles in an article formed by spray-forming is that it can serve as the locus from which cracks develop and spread. It is generally well recognized that a foreign material such as a particle of ceramic can serve as the focal point around which cracking develops in an article manufactured for use under high stress conditions. Such high stress may occur for example if the particle is embedded in a moving part of an aircraft engine where the part may rotate at speeds of 12,000 revolutions per minute or more. For stationary or static parts of apparatus and those which are subjected to low stress, the crack formation and propagation is not as great a danger. However the problem is that it is difficult in a ceramic lined system to determine just when the ceramic flake or particle will separate from the container and enter the stream. For this and other reasons the quest for an ultra-clean melting system has been of concern to many researchers and metal suppliers and activity in this area during recent years has been increasing. This effort has been directed toward drastically reducing or eliminating crack initiation sites from parts in which a ceramic inclusion may be picked up in the melt cycle and carried through to a casting or to a spray-forming cycle.
It is recognized that ceramic inclusions tend to have a density which is lower than that of the host metal melt in which they are included. For this reason there is a benefit obtained in avoiding top pour processing of molten metal as the particles are more likely to be included in a stream emanating from the top of a crucible than one which emanates from the bottom. While the particles tend to congregate at the top of a melt the stirring action which may attend the flow of the melt or which may attend induction power supply may not allow all particles to remain on top of the melt. Also particles splintered from a cracked crucible or cement used to adhere the nozzle and crucible together may also be swept into the melt stream as it emerges from the crucible nozzle at the bottom of a crucible. For this reason what I have developed here is in effect a ceramicless melt system.
The Duriron Company, Inc., of Dayton, Ohio has published a paper in the Journal of Metals in September 1986 entitled "Induction Skull Melting of Titanium and Other Reactive Alloys" by D. J. Chronister, S. W. Scott, D. R. Stickle, D. Eylon and F. H. Froes. In this paper an induction melting crucible for reactive alloys is described and discussed. In this sense it may be said that through the Duriron Company a ceramicless melt system is available. The present invention provides a method and apparatus which is an alternative to and improvement over the skull melting method and apparatus of the Duriron Company.
The controlled atomization of a liquid stream of metal and its deposition on a substrate by a spray-forming process requires that the molten stream of metal pass through a nozzle with a predetermined fixed bore size